Modern day integrated chips comprise millions or billions of semiconductor devices formed on a semiconductor substrate (e.g., silicon). To improve functionality of integrated chips, the semiconductor industry has continually reduced the dimension of semiconductor devices to provide for integrated chips with small, densely populated devices. By forming integrated chips having small, densely populated devices, the speed of the semiconductor devices increases as the power consumption of devices decreases. However, as the density of integrated chip devices increases, the failure rate of integrated chips due to contamination also goes up. One cause of this increased failure rate is the exposure of a wafer to airborne molecular contaminants (e.g., dust particles in the air).
To minimize integrated chip failure due to contaminants, fabrication facilities take great care to limit exposure of wafers to contaminant particles. For example, modern day integrated chips are formed in clean rooms having a low level of environmental pollutants. For example, an ISO 1 cleanroom contains no particles having a size of greater than or equal to 0.5 um (in comparison to ambient air outside, which contains approximately 35,000,000 particles of that size per cubic meter). Integrated chips are also transported between tools in a clean room using closed wafer carriers that further decrease exposure of an integrated chip to contaminants.
Ideally, wafers will ship to external customers with no defects or contaminants. However, in reality, some nominal level of defects and/or contamination cannot be entirely avoided. To measure the level of defects and/or contamination present on each wafer, outgoing quality assurance (OQA) processes are used on the final wafers just before the wafers are shipped.